It has been proposed to tag all commercially produced explosives with a unique material that can be easily identified. Vapor taggants of considerable cleverness have been investigated. However, they have two strikes against them, namely the required cooperation of explosives manufacturers (common to any taggant program; explosives manufacturers are particularly resistant to introducing any substance that reduces the performance or reliability of the explosive), and the ease with which the system can be circumvented by appropriately sealing the bomb.
This second drawback can be circumvented if a taggant is used that can be detected by its nuclear properties, or by some other penetrating probe. I have previously suggested that explosives or detonators that have been partially deuterated are easily detected by irradiation with 4 MeV gammas. This takes advantage of the fact that deuterium has the second lowest threshold for (gamma,n), and has the additional advantage that explosive performance is not sacrificed, since deuterated X has the same chemistry as ordinary X. The estimated cost is of the order of 10 cents per detonator or stick of dynamite. This scheme is adaptable to area searches (when everyone has been evacuated), where, with the longer integration time permitted, it is possible to search a whole airplane at once. Unfortunately, all tagging schemes have the distinct disadvantage that they have no applicability to scenarios that are driven by forces of international terrorism.
With almost no exceptions, all explosives current in use contain large amounts of nitrogen, typically between 20% and 35% by weight. Although there are some common articles that also contain nitrogen (animal products and some synthetics), they generally have nitrogen present in lower concentrations and being generally more spread out.
A known method exploits the nuclear reaction produced by the capture of slow neutrons by nitrogen nuclei, giving off an unusually high energy (10.8 MeV) gamma ray that is easily detected by scintillation detectors. The parcel to be examined passes through a shielded enclosure in which it is subject to slow neutrons while being examined for gamma emission. In order to "see" whether the source of gamma rays is compact (a bomb) or spread out (a nylon sweater), a number of detectors are used to form a crude image of the gamma emitting object, i.e., the shape of the nitrogen-containing material. It must always be crude, since slow neutrons do not go on straight lines but diffuse through the package being examined.
As additional related prior art, I discovered .sup.12 N and observed its then record-holding short half life (12 ms) in about 1949. The decay of this isotope produces back to back 511 keV annihilation radiation. Nitrogen 12, Physical Review, 1949.
The reaction .sup.14 N(gamma, 2n).sup.12 N was seen by Panofsky et al. in about 1952. This reaction has received little, if any, attention since discovery in so far as I am aware.